Making sure things don’t go ‘crack’

By Eamonn Ryan

A good geotechnical investigation is a bit like ‘cheap’ insurance for the cost and integrity of your structure. Geotechnical aspects of a project like piling and lateral support do go wrong – and when they do it’s significant and extremely costly. Where it more frequently goes wrong is in cost overrun – sometimes considerably.

No contractor wants to see an adjacent 17-floor building full of people collapse into their recently dug basement excavation, due to faults with the piling and lateral support. It’s not neighbourly.

The client is responsible for seeing that a geotechnical report is undertaken and supplied to the foundation contractor for design and pricing. It should be undertaken as early as possible before proceeding with a development as it is fundamental to the design and viability of the entire project. It may not even be an exorbitant expense – it may be as low as R100 000 to R150 000 for a geotechnical investigation in straight-forward geology for large multi-level office development in Sandton or Rosebank – and it saves money in the long run.

However, it is often not done properly. Shaun Nell, MD of geotechnical contractor Terra Strata Construction, says a way to resolve this is for civil and structural engineers to play a larger role in developments by laying down engineering parameters at the time of tender rather than have decisions made purely on commercial considerations.

Lesotho

Geotechnical investigations are currently underway in Lesotho for the Lesotho Highlands Water Project. Photo by JG Afrika.

“Or the quantity surveyor should advise the client of the appropriate standards of what is required for a project before going out to tender. These professionals are closely involved in the costing and feasibility. The contractor will never be able to influence the client’s thinking during the feasibility and tender stages – but the consulting professionals can.”

Trevor Green, geotechnical engineer and head of the Geotechnical Department at Jones & Wagner, says having the geotechnical report early enables better decisions to be taken on the development by the technical team.

“You know where the rock is, how easy it is to get the rock out. Done the other way round, you may be planning a multi-storey building with basements and only when you’ve dug out the basement do you find out the rock is inconveniently placed. This entirely changes the costing, affecting the viability of the project. Poor information also makes pricing difficult: is piling a R1-million or a R10-million portion of the budget?”

Green says his biggest fear in this regard is with lateral support. A lateral support failure can have catastrophic consequences and could easily result from poor or inappropriate geotechnical information being used for the design.

When an engineer receives a poor geotechnical report on which to base the design, that design is consequently based on assumptions. The design is then qualified with caveats and provisos based on the fact it isn’t clear what is below the ground. Piling contractors experience problems usually because of incomplete information in the geotechnical report due to an insufficient number of auger holes or boreholes being drilled, or drilled to insufficient depth, or only a portion of the site was investigated.

“There are some sites where the geology and geotechnical conditions vary dramatically. Johannesburg CBD is one such case, with an extremely weathered zone running through the middle of the CBD called the Grabon which can make it a horrible place to build. The rock can be up to 50m depth with weathered rubbish on top,” says Green.

Another common error is for people to make assumptions rather than investigate. “They’ve done work on the neighbouring site and assume the geology will be the same. Each individual site needs a separate site-specific and thorough geotechnical investigation because geologies do vary. We don’t know everything, and oddities can pitch up for no reason.”

In Sandton the geology is typically made up of granite with dolerite intrusions caused by ancient volcanic activity. In Pretoria East, you often find shallow shale rock intruded by much more deeply weathered dolerite. You cannot predict exactly where and how extensive the intrusions are, explains Green. The effect of this variation stems from the fact that different rocks weather at different rates. “We had one site in Pretoria where one half of a site was rock at three or four metres and the other half residual dolerite clay down to 20m, with literally a line drawn half way through the site. Each side required a completely different treatment in terms of lateral support and foundations.”

Piles can go as deep as 80m in extreme cases such as some of the massive buildings being built in Dubai, but in South Africa 20m to 30m is regarded as deep. The cost of piling to such depths, however, rises exponentially, says Green – hence the impact on costing.

limited access

Limited access drilling. Photo by Construction Drilling.

Cutting costs in the wrong places

To try save on costs is admirable, especially in such tough economic times, but Green cautions that often people try to save on costs in the wrong places. The geotechnical report is one instance, because it amounts to a tiny percentage of the total cost of a project, particularly when you factor in the full lifecycle costs of a project including maintenance. If something goes wrong for such a small overall saving, it will, in hindsight, have been an extremely poor decision. 

“Geotechnical reports are not always widely understood or appreciated, and of late appear to have come to be thought of as an area which can be readily cut to make savings by skimping on the geotechnical report or hiring someone cheap. Geotechnical investigations are all about what’s below the ground, and the only time some people even think about this aspect of a project is when something goes wrong,” says Green. However, this is when things get really expensive.

“It’s often open to interpretation what is required: I might think three boreholes are required while someone else says two will suffice and a third person wants 10. It’s highly subjective, and in every industry there are people who will come in cheap to get work. The difficulty with geotechnical investigations is that it’s difficult to quantify the benefits up front – on three jobs the client may be lucky and have no problems even with sub-standard geotechnical investigations, but on the fourth project it may be a complete disaster, more than wiping out the benefit of the other three. At that point everybody says they wished they’d spent more money on the geotechnical report.

“The business case for a proper geotechnical investigation is that the development goes ahead smoothly with no cost or time overruns. There was a development in Sandton recently where the contractor dug the basement, and based on the geotechnical report was expecting rock at basement level. The project was costed based on straightforward conventional foundations on rock. When this turned out not to be the case, they ended up over-excavating and pouring millions of rands of mass concrete into that basement to fill up softer areas and allow the original foundation solution to be implemented. This slowed them up by months. There can be severe penalties for time overruns and these add up quickly. This can also result in claims by other contractors,” says Green.

“For a typical development, the professional team and contractor costs and programmes the works according to the geotechnical information – for instance, if 30 000m3 of rock was expected to be moved and becomes 80 000m3 and takes an extra three months for the earthworks, this has massive financial implications and causes huge fights.” The client can often eliminate this risk by paying for a decent geotechnical investigation.

Geotechnical investigations in the rest of Africa are not theoretically any different to South Africa, but Green notes they are extremely difficult for logistical reason. Facilities such as quality commercial labs, which are readily available in South Africa, are typically scarce, and shipping large amounts of soil down to a lab in South Africa can be problematic at border customs. The same obstacles apply to getting work permits and equipment into the country. Jones & Wagner has done investigations in much of Africa, but does rank some countries as ‘virtually impossible’ due to safety concerns, excessive logistical difficulties and infrastructure problems.

Design and construct

In the 1970s and 1980s it was common for a client contemplating a development to hire an engineer to design everything – including foundations and building – and put it out to tender to be priced according to the engineer’s design. In that model, the engineer had complete control over the whole process. Later, specialisation became more commonplace and geotechnical contractors came up with their own designs at a cheaper price, meaning they then had to take full responsibility for that aspect of the work. Geotechnical contracting has been heavily biased towards this model over the past 25 years, though there are signs of it coming a little more into balance in recent years.

“Design and construct type contracts come with their own challenges – because you’re now designing something with a commercial incentive to win a tender in an environment where there is not a lot of work. Therefore, contractors and their designers start pushing the design right to the ragged edge. As designs get pushed further and further to save costs, you’re bound to get some problems as everyone is incentivised to take more risks. Arguably, an engineer designing directly for the client has less commercial pressure and would find it easier to build to appropriate levels of safety,” says Green. 

It’s easy to find examples of the sorts of things that can go wrong when the envelope is pushed too far, such as under-designing lateral support resulting in cracks in neighbouring buildings or under-designed piled foundations resulting in settlement of the structure. Either way, small savings do not justify the risk, especially when it comes to anything geotechnical. 

When a proper geotechnical investigation report is done, the blame for any subsequent problem may be laid at the door of the engineer or contractor. But if poor or inappropriate investigation is done the client has limited recourse, as the engineer or contractors have the right to claim they were working with faulty information. This is the perfect recipe for contractual claims, legal claims and counter-claims.

Piling’s perspective

Tera Strata’s Nell says his firm does design and construct projects most of the time and the relationship between the geotechnical report and piling and lateral support is ‘100% symbiotic’. Geotechnical construction companies do not generally do geotechnical reports; mostly they just provide the drilling equipment necessary.

“We need the geotechnical report to do the design and to assess the risks and methods suitable for the project. For certain conditions you will have certain methods of drilling, and this has to be determined prior to going to site.” It also enables the contractor to give the client a price which is not ‘qualified’. Nell says that in most cases he sees there is a substandard geotechnical report, even for large developments. This means the piling contractor has to qualify his bid price, which can get ‘unpleasant’ he says, when conditions underground prove to be different to what was expected based on information available.

In eight cases out of 10, there may be no problem – but in the other two cases out of 10 the blowout on costs makes it worthwhile to have done reports on all 10, he says.

“The law today requires a client to have a certain level of geotechnical report done, but the client typically does the minimum – to try to keep the building cost to an absolute minimum. The saying in the industry is that you pay for a proper report either way – at the beginning of the project or during construction.

“If a developer does the minimum report, then the projects will have a higher risk profile and the bidding contractor will take cognisance of that risk. We all have a good general idea of what is where – we know that Sandton is on granite; Rosebank is the same but a bit deeper; Centurion is on dolomite; Pretoria is shale and some other andesites – what we don’t know is what the specific conditions of the project are, as there can be significant variability. Therefore, we qualify our quotes. We put in a qualified design, a re-measurable bill of quantities and a method qualification. We would say things like ‘we did not allow for sidewall collapse’, or we priced CFA piles taking cognisance of water and so on. We do the best we can with what we’ve got, and qualify it.

“If it goes well and the qualifications were not required, then the client is happy because he’s saved a bit of money. If the site has conditions different to what we assumed, we may have to bring on additional equipment and that causes cost escalations, contractual claims and time delays,” explains Nell. When the client has to pay extra money, it becomes quite unpleasant and often evolves into a blame game.

“The client will argue that we’re the experts and ‘should have known’. We will argue that we ideally would have been provided with a comprehensive geotechnical investigation. It builds up bad blood, and such disputes can give our industry a bad name – undeservedly.” It does help that geotechnical consulting firms such as Jones & Wagner have considerable institutional memory around the country with its more than 50-year history and in the event of their design, will bring the possible risks to the fore.

However, in tough times like now, contractors will try present the most economical possible proposal based on a pricing according to the most optimistic interpretation of site conditions provided in the geotechnical report. “The likelihood of things going wrong on site therefore becomes bigger.”

A common problem is to unexpectedly find a high water table or soil that collapses during drilling – so that when the drill is removed the walls fall in, which in some cases requires the CFA method, and in other cases the use of temporary sleeves, says Nell. “These are not issues that undermine the integrity of the structure, but they add to the cost and time of installation,” he adds.

Types of geotechnical investigations

The choice of tools and equipment for a geotechnical investigation primarily depends on the type of access to the site, the geology present, the loads that one is going to apply to the ground and the type of structure to be built, explains Dave Rossiter, non-executive chairman of the GeoGroup.

Specialised equipment is used to access mountains, lakes and rivers and specialised drilling methods and in situ testing is required for specific types of structures. Generally, the more weathered the ground, the deeper one will have to investigate until sold rock is found. “For instance, the Fourways area is primarily granitic; Centurion dolomitic; the East Rand lies generally on Ecca series shale; and the south predominantly on andesite lava – and each rock type has different characteristics.

“These different soil and rock types will have different characteristics which will determine how they are investigated. For instance, for lighter structures you can, in the generally shallow weathered granites, simply make use of a trailer mounted Dynamic probe super heavy (DPSH) rig to conduct a DPSH test to understand the bearing capacity of the soil. But in the dolomites, even for lighter structures, you will need information from greater depths to understand if there are cavities present due to the manner in which the dolomites weather,” says Rossiter.

A different proposition is posed by the lava rock in the south of Johannesburg, which weathers in spheroids, with large boulders sometimes interspersed in soft clay, ‘hard and soft alternating layers which can go on for about 30m, which is also a troublesome rock to build on because of its inconsistency, especially for dynamic loads’. 

“The dolomites are quite different in that there’s no consistency. Below a hard crust, there may be anything from a wet muddy cavity full of wad, to a fresh pinnacle of dolomite with a strength of 200Mpa or into a honeycomb mix of spoil and rock which can extend for as much as 80m below the surface,” he says.

This challenge would generally be met with some geophysics testing and backed up with the cheapest form of drilling such as air drilling or percussion drilling. This method involves pumping air from a compressor down a rod string and into a ‘down the hole hammer’ which hammers the rock and blows the sample back up to the surface.

Rossiter explains that for this investigation to be successful you must have information on sample return, air loss during drilling, water colour and speed of drilling as well as signs of sudden voids encountered while drilling. This can be best achieved by using the Jean Lutz (LT3) data collection system which collects a range of seven different parameters electronically during drilling. Manual recording of these parameters with a stopwatch is often misleading and confusing especially when cavities are present.

During a recent piling project for a bridge on the Berg River at Val de Vie estate near Paarl, Geopile Africa (part of the Geogroup), found the underlying targeted Malmesbury shale to be covered with about 7m of large rounded quartzite boulders, which without the combination of the Atlas Copco Symmetrix drilling and grouting system, they could not have piloted stable holes through the boulders to accommodate the 170mm diameter ductile iron piles. “This is the same method that was successfully used on the Gautrain geotechnical investigation project, to install PVC ground penetrating radar tubes for Bombela on the dolomite rock in Centurion,” he says.

“Geotechnical investigations for large span bridges or water extraction pipelines can often demand the use of jack-up platforms on which to drill from, over water. Geomechanics has conducted numerous geotechnical investigation in rivers and lakes over the past 30 years. Recently, the bridge over the Zambezi River at Kazangula was investigated successfully by Geomechanics using a jack-up platform designed and built by the company in its workshops in Lanseria near Johannesburg.

“In this case, holes of larger diameter than usual were drilled in the river bed to establish the founding conditions for the piers and abutments of the bridge. These holes were drilled with PQ core barrels which give a core size of 84.7mm and Lefranc permeability tests were conducted on the shore boreholes in the alluvium. The accuracy of the positioning of these holes was of utmost importance and a Trimble survey instrument was used to position the barge and drill rig within 20mm of the required position,” says Rossiter. 

Another recent investigation was conducted successfully on lake Albert in Uganda [see Infra Africa, page 10] when Geomechanics modified the same Jack-up platform to work in 11m of water and conducted CPTU testing and NWD4 rotary core drilling in the lake bed for a pipeline to supply water to the oil company’s central process facility. This facility will prepare crude oil to be pumped some 1 300km away to the sea at Tanga in Tanzania for export.

The full geotechnical investigation in Uganda and Tanzania has taken over two years to complete and involved drilling 15 holes in lake Albert, seven holes on the Nile river where the crude oil pipeline crosses the Nile river from the north, 60 holes at the central process facility area, numerous holes on the well pads, as well as investigating the ground conditions for the 1 300km pipeline through both Uganda and Tanzania.

Dam sites and hydro power schemes are generally located in mountainous areas and often require specialised equipment and methods to access the sites like river barges or helicopters and require specialised in situ testing to provide the necessary design information to the design engineers.

Geogroup also uses sonic drilling where necessary. Geomechanics has two sonic rigs both of which are mounted on rubber tracks to access rough terrain sites and to do so with minimal damage to the environment. Rossiter explains that this method involves high frequency resonant vibrations which are sent down the drill string to the drill bit, with the operator controlling the frequencies to suit the soil/rock geology.

“The resonance magnifies the amplitude of the drill bit, which fluidises the soil particles at the bit face, allowing for fast and easy penetration through most geological formations. An internal air spring isolates these vibrational forces from the rest of the rig, and by providing the necessary rotational and vibrational forces, the sonic rig is able to core and case holes in any overburden material, drilling where most other rigs can't. The cores are held in the core barrel by friction and/or by the use of core catchers as required, and core samples are gently extruded by vibration typically into plastic sleeves. Compressed air is available to assist core extraction if necessary,” he says.

There are a number of benefits to sonic drilling, though they are extremely expensive. Rossiter describes it as better suited to softer ground formations such as dune sands where mineral sands will be investigated or in granular alluvial ground.

He lists the advantages as follows:

  • Superior information.
    o    Continuous, relatively undisturbed core sample through any type of formation.
    o  Continuous core samples to depths of more than 100m with or without using any drilling mud.
  • It is two to three times faster.
  • Superior well construction causing minimal disturbance to the surrounding borehole wall.
  • Flexibility – it advances a temporary outer casing as the borehole is drilled, allowing you to do more within a single borehole.
  • Risk minimisation.
    o    Reduces the risk of project failure due to unknown or difficult subsurface conditions
    o    Recovery rate in excess of 95%
    o    Projects finish on time and on budget
    o    Obtains the lowest total project cost possible. 
Dave Rossiter Shaun Nell Trevor Green2

 

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